1. Field of the Invention
This invention relates to a metal gasket having a seal structure for three-surface combining portions, comprising metal plates adapted to seal clearances between opposed surfaces of a three-surface combining portion formed of parts of, for example, a chain case-carrying cylinder block and a cylinder head.
2. Description of the Prior Art
There is a known metal gasket for air-tightly and liquid-tightly retaining clearances between upper surfaces of a cylinder block and a chain case which are fixed to each other in a contacting state, and a lower surface of a cylinder head. This metal gasket is provided with a main seal portion for sealing a clearance around a combustion chamber which requires a high seal surface pressure, and an auxiliary seal portion for sealing a clearance around the chain case which serves the purpose with a comparatively low seal surface pressure. This metal gasket is formed by extending at least one of laminated thin metal plates, which form the main seal portion, so as to prepare the extended section as a base member of an auxiliary seal portion which serves the purpose with a low surface pressure, and bonding a soft sheet, such as a beater sheet, a graphite sheet or a compressed sheet to one or both surfaces of the base member (refer to, for example, Japanese Utility Model Laid-Open No. 48059/1988).
The metal gaskets applied to a chain case-carrying engine include the metal gasket disclosed in Japanese Patent Laid-Open No. 74659/1996. This metal gasket is capable of reliably preventing the entry of an oil into a stepped portion, which occurs on the opposed surfaces of a cylinder block and a chain case, even when an oil deposited on the chain case scatters during the traveling of a chain. This metal gasket comprises a first elastic metal plate provided with beads along the circumferences of openings through which a camshaft driving chain travels, an intermediate plate having holes therein, and a second elastic metal plate laminated on the intermediate plate. The intermediate plate is provided with seal member for sealing a stepped portion occurring on opposed surfaces of the chain case and cylinder block. The second elastic metal plate is provided with cutout portions in the part thereof which corresponds to this seal member. The holes of the intermediate plate extend from opposed surfaces toward the chain case.
Japanese Utility Model Laid-Open No. 148962/1986 discloses a gasket for a three-surface combining portion. This gasket is applied to a three-surface combining portion in which a bead-carrying gasket is held between first and second members with a liquid sealant inserted between the resultant product and a third member, a seal member which is other than the liquid sealant being packed in a bead in an end section of the gasket in the three-surface combining portion.
In OHC type and DOH type engines, a chain case 19 for holding a camshaft driving chain therein is generally formed separately from a cylinder block 18 as shown in FIG. 10, which chain case 19 provided with a passage 23 is set adjacently to a cylinder block 18, in which a cylinder, i.e. a combustion chamber 25 is provided, and combined therewith by bolts.
A metal gasket is provided between a cylinder head and a chain case 19 and a cylinder block 18. Referring to the drawing, a reference numeral 22 denotes bolt holes. Moreover, in order to reduce the weight of an engine, a fixing side surface of the chain case is formed in an opened state, whereby the weight of the chain case is reduced, the chain traveling passage 23 being formed by an inner surface of the passage 23 in the chain case 19 and a side surface of the cylinder block 18.
In this engine, separate parts are fixed together. Therefore, even when an upper surface 24 of the cylinder block 18 and that 21 of the chain case 19 are finished horizontally, it cannot be avoided that a very small height difference 20 occurs at the portions of the two upper surfaces which are at the upper ends of joint surfaces, i.e. opposed surfaces 26 of the cylinder block 18 and chain case 19. It cannot also be avoided that the height difference 20 increases while the engine is used for a long period of time. When the height difference 20 occurs between these two upper surfaces, a lubricating oil flies in accordance with the traveling of the camshaft driving chain in the chain case 19, and is deposited on the height different portion. The resultant lubricating oil enters the height different portion, and leaks out to the exterior of the chain case or flows into a clearance between the cylinder block 18 and cylinder head.
In this metal gasket, a structure formed by extending the portion of a thin metal plate which is laminated on the side of the cylinder block 18 to prepare this portion as a base member, and bonding a soft sheet to one surface thereof cannot retain the air-tightness and liquid-tightness of the height different portion 20. In a structure formed by extending an intermediately positioned thin metal plate to prepare this extended portion as a base member, and bonding soft sheets to both surfaces thereof, the air-tightness of the height different portion 20 can be retained when the height difference is very small. When the height difference 20 becomes large, a larger thickness of the soft sheets is required to secure the sealing performance of the structure, and it becomes necessary to design the lamination of metal plates with the thickness of a main seal portion and that of these soft sheets taken into consideration. In general, a beater sheet, a graphite sheet and a compressed sheet which constitute a soft sheet has a thickness of at least around 0.5 mm originally, so that it is necessary to set the thickness of the main seal portion in accordance with that of the soft sheet. Therefore, in order to secure a high surface pressure on the main seal portion, the number of thin metal plates to be laminated has to be increased.
In the metal gasket disclosed in the above-mentioned Japanese Patent Laid-Open No. 74659/1996, a seal member is provided separately from the intermediate plate, and a part of an elastic metal plate is cut out so as to provide the seal member therein, the seal member being set practically in the cutout portion. Accordingly, it is necessary to make a cutout portion in the elastic metal plate, and set a seal member stably in the cutout portion. Since the seal member is formed independently and set in the hole of the intermediate plate, it is restricted by the number of the metal plates and the thickness of the intermediate plate in the laminated structure. This metal gasket requires the intermediate plate boring work and elastic metal plate cutout work, so that it becomes inadvantageous with respect to the manufacturing cost including the assembling manday.
Regarding an engine using the chain case-carrying metal gasket, the lower surface of the cylinder head is flat and continuous. When a height difference occurs between the upper surface of the cylinder block and that of the chain case, a method of preventing the entry of an oil in the chain case into the height different portion has to be devised without employing a means, which causes the gasket manufacturing cost to increase, for increasing the number of thin metal plates to be laminated.